1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of Related Art
Typical electrophotographic image forming apparatuses correct the gradation characteristic of image data so that the gradation characteristic of the image agrees with a target gradation characteristic. To correct the gradation characteristic, a gradation correction table is used in which corrected gradation levels are specified for respective gradation levels.
Since the gradation characteristic of an image changes along with a change in environment and a lapse of time, image forming systems periodically adjusts (calibrates) the gradation characteristic so as to optimize the gradation correction table. The calibration process involves forming a gradation adjustment mark on a sheet, reading the image of the gradation adjustment mark by means of a scanner or the like, and newly forming or updating the gradation correction table based on the difference between the gradation characteristic specified from the read gradation adjustment mark and the target gradation characteristic.
For example, such gradation adjustment marks are formed in a non-image area, which is out of an image area where the image of a job is formed (e.g. see JP 2014-107648A). This eliminates the need for forming a gradation adjustment mark on another sheet other than the sheet on which a user required image is formed. Therefore, it is not required to interrupt an ongoing job, and a waste paper is not produced.